Kotlin/Native as an Apple framework – tutorial

Kotlin/Native provides bi-directional interoperability with Objective-C/Swift. Objective-C frameworks and libraries can be used in Kotlin code. Kotlin modules can be used in Swift/Objective-C code too. Besides that, Kotlin/Native has C Interop. There is also the Kotlin/Native as a Dynamic Library tutorial for more information.

In this tutorial, you will see how to use Kotlin/Native code from Objective-C and Swift applications on macOS and iOS.

In this tutorial you'll:

create a Kotlin Library and compile it to a framework
examine the generated Objective-C and Swift API code
use the framework from Objective-C and Swift
Configure Xcode to use the framework for macOS and iOS

Create a Kotlin library

The Kotlin/Native compiler can produce a framework for macOS and iOS out of the Kotlin code. The created framework contains all declarations and binaries needed to use it with Objective-C and Swift. The best way to understand the techniques is to try it for ourselves. Let's create a tiny Kotlin library first and use it from an Objective-C program.

Create the hello.kt file with the library contents:

package example

object Object {
  val field = "A"
}

interface Interface {
  fun iMember() {}
}

class Clazz : Interface {
  fun member(p: Int): ULong? = 42UL
}

fun forIntegers(b: Byte, s: UShort, i: Int, l: ULong?) { }
fun forFloats(f: Float, d: Double?) { }

fun strings(str: String?) : String {
  return "That is '$str' from C"
}

fun acceptFun(f: (String) -> String?) = f("Kotlin/Native rocks!")
fun supplyFun() : (String) -> String? = { "$it is cool!" }

While it is possible to use the command line, either directly or by combining it with a script file (such as .sh or .bat file), this approach doesn't scale well for big projects that have hundreds of files and libraries. It is therefore better to use the Kotlin/Native compiler with a build system, as it helps to download and cache the Kotlin/Native compiler binaries and libraries with transitive dependencies and run the compiler and tests. Kotlin/Native can use the Gradle build system through the kotlin-multiplatform plugin.

We covered the basics of setting up an IDE compatible project with Gradle in the A Basic Kotlin/Native Application tutorial. Please check it out if you are looking for detailed first steps and instructions on how to start a new Kotlin/Native project and open it in IntelliJ IDEA. In this tutorial, we'll look at the advanced C interop related usages of Kotlin/Native and multiplatform builds with Gradle.

First, create a project folder. All the paths in this tutorial will be relative to this folder. Sometimes the missing directories will have to be created before any new files can be added.

Use the following build.gradle(.kts) Gradle build file:

plugins {
    kotlin("multiplatform") version "2.1.0"
}

repositories {
    mavenCentral()
}

kotlin {
  macosX64("native") {
    binaries {
      framework {
        baseName = "Demo"
      }
    }
  }
}

tasks.wrapper {
  gradleVersion = "8.10"
  distributionType = Wrapper.DistributionType.ALL
}

plugins {
    id 'org.jetbrains.kotlin.multiplatform' version '2.1.0'
}

repositories {
    mavenCentral()
}

kotlin {
  macosX64("native") {
    binaries {
      framework {
        baseName = "Demo"
      }
    }
  }
}

wrapper {
  gradleVersion = "8.10"
  distributionType = "ALL"
}

Move the sources file into the src/nativeMain/kotlin folder under the project. That is the default path, where sources are located, when the kotlin-multiplatform plugin is used. Use the following block to configure the project to generate a dynamic or shared library:

binaries {
  framework {
    baseName = "Demo"
  }  
}

Along with macOS X64, Kotlin/Native supports macos arm64 and iOS arm32, arm64 and X64 targets. You may replace the macosX64 with respective functions as shown in the table:

Target platform/device	Gradle function
macOS x86_64	`macosX64()`
macOS ARM 64	`macosArm64()`
iOS ARM 64	`iosArm64()`
iOS Simulator (x86_64)	`iosX64()`
iOS Simulator (arm64)	`iosSimulatorArm64`

Run the linkNative Gradle task to build the library in the IDE or by calling the following console command:

./gradlew linkNative

Depending on the variant, the build generates the framework into the build/bin/native/debugFramework and build/bin/native/releaseFramework folders. Let's see what is inside.

Generated framework headers

Each of the created frameworks contains the header file in <Framework>/Headers/Demo.h. The headers do not depend on the target platform (at least with Kotlin/Native v.0.9.2). It contains the definitions for our Kotlin code and a few Kotlin-wide declarations.

Kotlin/Native runtime declarations

Take a look at Kotlin runtime declarations:

NS_ASSUME_NONNULL_BEGIN

@interface KotlinBase : NSObject
- (instancetype)init __attribute__((unavailable));
+ (instancetype)new __attribute__((unavailable));
+ (void)initialize __attribute__((objc_requires_super));
@end;

@interface KotlinBase (KotlinBaseCopying) <NSCopying>
@end;

__attribute__((objc_runtime_name("KotlinMutableSet")))
__attribute__((swift_name("KotlinMutableSet")))
@interface DemoMutableSet<ObjectType> : NSMutableSet<ObjectType>
@end;

__attribute__((objc_runtime_name("KotlinMutableDictionary")))
__attribute__((swift_name("KotlinMutableDictionary")))
@interface DemoMutableDictionary<KeyType, ObjectType> : NSMutableDictionary<KeyType, ObjectType>
@end;

@interface NSError (NSErrorKotlinException)
@property (readonly) id _Nullable kotlinException;
@end;

Kotlin classes have a KotlinBase base class in Objective-C, the class extends the NSObject class there. There are also wrappers for collections and exceptions. Most of the collection types are mapped to similar collection types from the other side:

Kotlin	Swift	Objective-C
List	Array	NSArray
MutableList	NSMutableArray	NSMutableArray
Set	Set	NSSet
Map	Dictionary	NSDictionary
MutableMap	NSMutableDictionary	NSMutableDictionary

Kotlin numbers and NSNumber

The next part of the <Framework>/Headers/Demo.h contains number type mappings between Kotlin/Native and NSNumber. There is the base class called DemoNumber in Objective-C and KotlinNumber in Swift. It extends NSNumber. There are also child classes per Kotlin number type:

Kotlin	Swift	Objective-C	Simple type
`-`	`KotlinNumber`	`<Package>Number`	`-`
`Byte`	`KotlinByte`	`<Package>Byte`	`char`
`UByte`	`KotlinUByte`	`<Package>UByte`	`unsigned char`
`Short`	`KotlinShort`	`<Package>Short`	`short`
`UShort`	`KotlinUShort`	`<Package>UShort`	`unsigned short`
`Int`	`KotlinInt`	`<Package>Int`	`int`
`UInt`	`KotlinUInt`	`<Package>UInt`	`unsigned int`
`Long`	`KotlinLong`	`<Package>Long`	`long long`
`ULong`	`KotlinULong`	`<Package>ULong`	`unsigned long long`
`Float`	`KotlinFloat`	`<Package>Float`	`float`
`Double`	`KotlinDouble`	`<Package>Double`	`double`
`Boolean`	`KotlinBoolean`	`<Package>Boolean`	`BOOL/Bool`

Every number type has a class method to create a new instance from the related simple type. Also, there is an instance method to extract a simple value back. Schematically, declarations look like that:

__attribute__((objc_runtime_name("Kotlin__TYPE__")))
__attribute__((swift_name("Kotlin__TYPE__")))
@interface Demo__TYPE__ : DemoNumber
- (instancetype)initWith__TYPE__:(__CTYPE__)value;
+ (instancetype)numberWith__TYPE__:(__CTYPE__)value;
@end;

Where __TYPE__ is one of the simple type names and __CTYPE__ is the related Objective-C type, for example, initWithChar(char).

These types are used to map boxed Kotlin number types into Objective-C and Swift. In Swift, you may simply call the constructor to create an instance, for example, KotlinLong(value: 42).

Classes and objects from Kotlin

Let's see how class and object are mapped to Objective-C and Swift. The generated <Framework>/Headers/Demo.h file contains the exact definitions for Class, Interface, and Object:

NS_ASSUME_NONNULL_BEGIN

__attribute__((objc_subclassing_restricted))
__attribute__((swift_name("Object")))
@interface DemoObject : KotlinBase
+ (instancetype)alloc __attribute__((unavailable));
+ (instancetype)allocWithZone:(struct _NSZone *)zone __attribute__((unavailable));
+ (instancetype)object __attribute__((swift_name("init()")));
@property (readonly) NSString *field;
@end;

__attribute__((swift_name("Interface")))
@protocol DemoInterface
@required
- (void)iMember __attribute__((swift_name("iMember()")));
@end;

__attribute__((objc_subclassing_restricted))
__attribute__((swift_name("Clazz")))
@interface DemoClazz : KotlinBase <DemoInterface>
- (instancetype)init __attribute__((swift_name("init()"))) __attribute__((objc_designated_initializer));
+ (instancetype)new __attribute__((availability(swift, unavailable, message="use object initializers instead")));
- (DemoLong * _Nullable)memberP:(int32_t)p __attribute__((swift_name("member(p:)")));
@end;

The code is full of Objective-C attributes, which are intended to help the use of the framework from both Objective-C and Swift languages. DemoClazz, DemoInterface, and DemoObject are created for Clazz, Interface, and Object respectively. The Interface is turned into @protocol, both a class and an object are represented as @interface. The Demo prefix comes from the -output parameter of the kotlinc-native compiler and the framework name. You can see here that the nullable return type ULong? is turned into DemoLong* in Objective-C.

Global declarations from Kotlin

All global functions from Kotlin are turned into DemoLibKt in Objective-C and into LibKt in Swift, where Demo is the framework name and set by the -output parameter of kotlinc-native.

NS_ASSUME_NONNULL_BEGIN

__attribute__((objc_subclassing_restricted))
__attribute__((swift_name("LibKt")))
@interface DemoLibKt : KotlinBase
+ (void)forIntegersB:(int8_t)b s:(int16_t)s i:(int32_t)i l:(DemoLong * _Nullable)l __attribute__((swift_name("forIntegers(b:s:i:l:)")));
+ (void)forFloatsF:(float)f d:(DemoDouble * _Nullable)d __attribute__((swift_name("forFloats(f:d:)")));
+ (NSString *)stringsStr:(NSString * _Nullable)str __attribute__((swift_name("strings(str:)")));
+ (NSString * _Nullable)acceptFunF:(NSString * _Nullable (^)(NSString *))f __attribute__((swift_name("acceptFun(f:)")));
+ (NSString * _Nullable (^)(NSString *))supplyFun __attribute__((swift_name("supplyFun()")));
@end;

You see that Kotlin String and Objective-C NSString* are mapped transparently. Similarly, Unit type from Kotlin is mapped to void. We see primitive types are mapped directly. Non-nullable primitive types are mapped transparently. Nullable primitive types are mapped into Kotlin<TYPE>* types, as shown in the table above. Both higher order functions acceptFunF and supplyFun are included, and accept Objective-C blocks.

More information about all other types mapping details can be found in the Objective-C Interop documentation article

Garbage collection and reference counting

Objective-C and Swift use reference counting. Kotlin/Native has its own garbage collection too. Kotlin/Native garbage collection is integrated with Objective-C/Swift reference counting. You do not need to use anything special to control the lifetime of Kotlin/Native instances from Swift or Objective-C.

Use the code from Objective-C

Let's call the framework from Objective-C. For that, create the main.m file with the following content:

#import <Foundation/Foundation.h>
#import <Demo/Demo.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        [[DemoObject object] field];
        
        DemoClazz* clazz = [[ DemoClazz alloc] init];
        [clazz memberP:42];
        
        [DemoLibKt forIntegersB:1 s:1 i:3 l:[DemoULong numberWithUnsignedLongLong:4]];
        [DemoLibKt forIntegersB:1 s:1 i:3 l:nil];
        
        [DemoLibKt forFloatsF:2.71 d:[DemoDouble numberWithDouble:2.71]];
        [DemoLibKt forFloatsF:2.71 d:nil];
        
        NSString* ret = [DemoLibKt acceptFunF:^NSString * _Nullable(NSString * it) {
            return [it stringByAppendingString:@" Kotlin is fun"];
        }];
        
        NSLog(@"%@", ret);
        return 0;
    }
}

Here you call Kotlin classes directly from Objective-C code. A Kotlin object has the class method function object, which allows us to get the only instance of the object and to call Object methods on it. The widespread pattern is used to create an instance of the Clazz class. You call the [[ DemoClazz alloc] init] on Objective-C. You may also use [DemoClazz new] for constructors without parameters. Global declarations from the Kotlin sources are scoped under the DemoLibKt class in Objective-C. All methods are turned into class methods of that class. The strings function is turned into DemoLibKt.stringsStr function in Objective-C, you can pass NSString directly to it. The return is visible as NSString too.

Use the code from Swift

The framework that you compiled with Kotlin/Native has helper attributes to make it easier to use with Swift. Convert the previous Objective-C example into Swift. As a result, you'll have the following code in main.swift:

import Foundation
import Demo

let kotlinObject = Object()
assert(kotlinObject === Object(), "Kotlin object has only one instance")

let field = Object().field

let clazz = Clazz()
clazz.member(p: 42)

LibKt.forIntegers(b: 1, s: 2, i: 3, l: 4)
LibKt.forFloats(f: 2.71, d: nil)

let ret = LibKt.acceptFun { "\($0) Kotlin is fun" }
if (ret != nil) {
  print(ret!)
}

The Kotlin code is turned into very similar looking code in Swift. There are some small differences, though. In Kotlin any object has only one instance. Kotlin object Object now has a constructor in Swift, and we use the Object() syntax to access the only instance of it. The instance is always the same in Swift, so that Object() === Object() is true. Methods and property names are translated as-is. Kotlin String is turned into Swift String too. Swift hides NSNumber* boxing from us too. We can pass a Swift closure to Kotlin and call a Kotlin lambda function from Swift too.

More documentation on the types mapping can be found in the Objective-C Interop article.

Xcode and framework dependencies

You need to configure an Xcode project to use our framework. The configuration depends on the target platform.

Xcode for macOS target

First, in the General tab of the target configuration, under the Linked Frameworks and Libraries section, you need to include our framework. This will make Xcode look at our framework and resolve imports both from Objective-C and Swift.

The second step is to configure the framework search path of the produced binary. It is also known as rpath or run-time search path. The binary uses the path to look for the required frameworks. We do not recommend installing additional frameworks to the OS if it is not needed. You should understand the layout of your future application, for example, you may have the Frameworks folder under the application bundle with all the frameworks you use. The @rpath parameter can be configured in Xcode. You need to open the project configuration and find the Runpath Search Paths section. Here you specify the relative path to the compiled framework.

Xcode for iOS targets

First, you need to include the compiled framework in the Xcode project. To do this, add the framework to the Frameworks, Libraries, and Embedded Content section of the General tab of the target configuration page.

The second step is to then include the framework path into the Framework Search Paths section of the Build Settings tab of the target configuration page. It is possible to use the $(PROJECT_DIR) macro to simplify the setup.

The iOS simulator requires a framework compiled for the ios_x64 target, the iOS_sim folder in our case.

This Stackoverflow thread contains a few more recommendations. Also, the CocoaPods package manager may be helpful to automate the process too.

Kotlin/Native has bidirectional interop with Objective-C and Swift languages. Kotlin objects integrate with Objective-C/Swift reference counting. Unused Kotlin objects are automatically removed. The Objective-C Interop article contains more information on the interop implementation details. Of course, it is possible to import an existing framework and use it from Kotlin. Kotlin/Native comes with a good set of pre-imported system frameworks.

Kotlin/Native supports C interop too. Check out the Kotlin/Native as a Dynamic Library tutorial for that.

Last modified: 25 September 2024